Digital Optical Modulator for Programmable n-Quadrature Amplitude Modulation Generation

ABSTRACT

An optical transceiver comprising an optical signal input, a first modulation section coupled to the optical signal input, a second modulation segment coupled to the optical signal input and positioned in serial with the first modulation section, wherein the first modulation section comprises a first digital electrical signal input, a first digital driver coupled to the first digital electrical signal input, and a first modulator coupled to the first digital driver, and wherein the second modulation section comprises a second digital electrical signal input, a second digital driver coupled to the second digital electrical signal input, and a second modulator coupled to the second digital driver, and an optical signal output coupled to the first modulation section and the second modulation section.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Quadrature amplitude modulation (QAM) is a modulation scheme thatconveys two signals by modulating the amplitudes of two carrier waves.The carrier waves, which are generally sinusoids, are known asquadrature carriers because they are out of phase with each other by 90degrees. The modulate waves are then summed and transmitted to adestination. QAM is used extensively in the telecommunications field,and is increasingly being used in optical fiber systems.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising adigital electrical signal input, an optical signal input, a digitaldriver coupled to the digital electrical signal input, and a modulatorcoupled to the optical signal input and the digital driver, whereinthere is no digital-to-analog converter positioned between the digitalelectrical signal input and the modulator.

In another embodiment, the disclosure includes an optical transceivercomprising an optical signal input, a first modulation section coupledto the optical signal input, a second modulation segment coupled to theoptical signal input and positioned in parallel with the firstmodulation section, wherein the first modulation section comprises afirst digital electrical signal input, a first digital driver coupled tothe first digital electrical signal input, and a first modulator coupledto the first digital driver, and wherein the second modulation sectioncomprises a second digital electrical signal input, a second digitaldriver coupled to the second digital electrical signal input, and asecond modulator coupled to the second digital driver, and an opticalsignal output coupled to the first modulation section and the secondmodulation section.

In yet another embodiment, the disclosure includes a method comprisingreceiving an optical signal, and modulating the optical signal using adigital electrical signal from a digital electrical signal processor,wherein the digital electrical signal is not converted to an analogsignal prior to being used to modulate the optical signal.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a traditionalapparatus that may be utilized for nQAM generation.

FIG. 2 is a schematic diagram of an embodiment of a digital opticaltransceiver that may be utilized for nQAM generation.

FIG. 3 is a graph of a 16QAM signal constellation according to anembodiment of this disclosure.

FIG. 4 is a graph of a 64QAM signal constellation according to anembodiment of this disclosure.

FIG. 5 is a graphical representation of an embodiment of 16QAMgeneration in one arm of a transceiver using three modulation sections.

FIG. 6 is a table presenting electrical field values for 16QAMgeneration using three sections according to an embodiment of thisdisclosure.

FIG. 7 is a schematic diagram illustrating delay in an embodiment of atransceiver that may be utilized for nQAM generation.

FIG. 8 is a table presenting electrical field values for an embodimentof a transceiver that may be utilized for nQAM generation.

FIG. 9 is a schematic diagram of an embodiment of a digital opticaltransceiver that may be utilized for nQAM generation.

FIG. 10 is a table presenting electrical field values for 16QAMgeneration using two unequal sections according to an embodiment of thisdisclosure.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

A recent trend in optical electronics has been the increasing use ofcoherent technology in 40 Gigabit-per-second (Gb/s) and 100 Gb/s and/orfaster systems. However, high power consumption and high cost associatedwith various key optical electronic parts, e.g. high-speedanalog-to-digital (ADC) and/or digital-to-analog (DAC) convertors,optical modulators, and linear radio frequency (RF) drivers, may beimpeding the implementation of components such as transceivers incoherent form. Accordingly, a low power modulator suitable for use inoptical electronics systems may be desirable.

Disclosed herein is a system for the generation of a plurality ofelectrical signal modulation formats using less complex hardware thanprior art designs. The system may employ a segmented phase modulator,which may utilize one or more digital drivers to facilitate thegeneration of the plurality of modulation formats, e.g.non-return-to-zero (NRZ) and/or n-quadrature amplitude modulation (nQAM)where n is an integer indicating the level or degree of modulation. Thesystem may also facilitate the modulation of optical signals without theneed for a linear driver and/or without the need for a DAC at the outputof an application specific integrated circuit (ASIC) generating a signalfor controlling the modulation. Thus, the disclosed system may reducethe power dissipation and cost conventionally associated with opticalmodulation.

FIG. 1 is a schematic diagram of an embodiment of a traditionalapparatus 100 that may be utilized for nQAM generation. Apparatus 100may include one or more DACs 110 (e.g. located inside an ASIC) coupledto one or more linear RF drivers 120, coupled to an I/Q modulator 130.The I/Q modulator 130 may receive an optical input signal and divide thesignal into two components—an I component which may be referred to asthe “in phase” component of the signal, and a Q component which may bereferred to as the quadrature component of the signal. The Q componentof the signal may experience a ninety-degree phase shift from the Icomponent of the signal. The DAC 110 may receive a N-bit (where N is aninteger and N>0) digital electrical signal from an ASIC, and convert theN-bit digital electrical signal to a corresponding analog signal. Thecorresponding analog signal may then be transmitted through the linearRF driver 120 to increase the signal's output power to a levelsufficient for powering the I/Q modulator 130 and achieving a reasonablepower level for the optical output signal. Each of the DACs 110 and thelinear RF drivers 120 may have a relatively high power dissipation, e.g.about 6 Watts (W) for four 40 nanometer (nm) process complimentarymetal-oxide semiconductor (CMOS) DACs and about 5 W to about 8 W for agallium arsenide (GaAs) based RF driver. The DACs 110 and linear RFdrivers 120 may also occupy a large amount of the relatively limitedspace available on a die for integrated optical electronics circuits.Additionally, each DAC 110 may have a limited bandwidth of about 15-20gigahertz (GHz) due to limitations in the 40 nm CMOS process used intheir creation, thereby limiting the overall bandwidth of apparatus 100to about 15-20 GHz. The high power consumption of the DACs 110 andlinear RF drivers 120 may make high-density multi-channel integrationdifficult to achieve.

FIG. 2 is a schematic diagram of an embodiment of a digital opticaltransceiver 200 that may be utilized for nQAM generation. Transceiver200 may comprise an optical signal input, an optical splitter, digitalelectrical signal inputs 210 a-n, logical inverters 220 a-n utilized asdrivers, a segmented optical phase I/Q modulator 230 (comprising an Isignal component modulation section 230 a and a Q signal componentmodulation section 230 b), a phase shifter 240, an optical combiner, andan optical signal output. The optical splitter and optical combiner maybe inherent internal components of a segmented phase modulator and areillustrated for purposes of clarity. Transceiver 200 may furthercomprise two arms or segments, one for modulation section 230 a and onefor modulation section 230 b. Each arm of transceiver 200 may comprise Nsections, where N is an integer and may be limited by the digitalelectrical signal output resolution of an ASIC generating digitalelectrical signal inputs 210 a-n. In some embodiments, a minimum numberof sections N that may be necessary for nQAM generation may bedetermined according to the equation: N=(n^(0.5))−1. Each section maycomprise a digital electrical signal input 210 coupled to one or morelogical inverters 220, each of which is in turn coupled to itsrespective modulation section 230 a or 230 b. The optical signal inputmay be divided into its respective I and Q signal components uponentering transceiver 200 by the optical splitter, after which the I andQ signal components may be transmitted to the corresponding modulationsections 230 a and 230 b. Digital electrical signal inputs 210 a-n maybe digital electrical signals that may be utilized to drive modulator230. Digital electrical signal inputs 210 a-n may further bedifferential signals that have a value of about +1 on a first side and avalue of about −1 on a second side. Digital electrical signal inputs 210a-n may be transmitted to logical inverters 220 a-n to increase thepower level of the signals prior to transmitting the signals to themodulator 230. Phase shifter 240 may be coupled to the output ofmodulation section 230 b and may shift the phase of the output signalfrom modulation section 230 b by 90 degrees. The optical combiner maycombine the output signal from phase shifter 240 with the output signalfrom modulation section 230 a to form the optical signal output.

Each arm of transceiver 200 may include isolation sections locatedbetween each active section to reduce crosstalk between the sections andimprove overall performance. Additionally, the insertion loss, length ofeach section, and Vπ may be optimized to facilitate the best possiblebandwidth output power. For example, for a transceiver with about 10sections, each having a length of about 0.5 millimeters (mm), the totalactive length may be about 5 mm. For an insertion loss of about 1decibel (dB)/mm, Vπ*L=1 volt (V)*centimeter (cm), and a fixed driveroutput of about 1 volt peak-to-peak (Vpp), the total insertion loss ofthe active region may be about 5 dB, Vπ=2V, and the modulation losscaused by under-modulating may be about 3 dB. For this example, thetotal loss with modulation may be about 8 dB (not including multi-modeinterface (MMI)). The logical inverters 220 a-n utilized as digitaldrivers may consume very little power. This may be because the logicalinverters 220 a-n may work in an on or off state without varyingdegrees, and, ideally, may have no current flowing through it. Forexample, an array of eight 40 nm CMOS inverters, at Vpp equal to about1V, may consume about 100 milli-watts (mW) power. A dual-polarization(DP) nQAM transceiver utilizing a four-driver array may consumer about400 mW of power, which may be a significant reduction in powerconsumption compared to that of apparatus 100. In a transceiver such astransceiver 200, the significant reduction in power consumption comparedto apparatus 100 may be realized through the elimination of DACs andlinear drivers. Additionally, the elimination of DACs may increase thebandwidth of a transceiver such as transceiver 200 to a value limitedonly by each individual section of the modulator rather than the totalcollective bandwidth of the modulator itself, or of the DACs. As such,transceiver 200 may not employ DACs or linear drivers in its modulationan optical signal.

Alternate embodiments of transceiver 200 may be formed by groupingnearby sections together and driving them simultaneously from the ASIC,thereby programming transceiver 200 to generate varying nQAM signals.For example, in a transceiver 200 utilizing seven sections, analternative embodiment may be formed by grouping two consecutive sets ofthree sections together to facilitate the generation of a 16QAMquadrature phase shifted keying (QPSK) signal. The remaining section maybe disabled, may be used as an extra bit for equalization, or may beused in another manner dictated by design choice. Using this techniqueof grouping various combinations of sections, embodiments of transceiver200 may be formed that may be capable of generating a plurality ofsignals, e.g. NRZ and/or nQAM, with the same hardware.

FIG. 3 is a graph 300 of a 16QAM signal constellation according to anembodiment of this disclosure. A 16QAM signal may be generated by atransceiver, such as transceiver 200 shown in FIG. 2, by using aboutthree equal sections according to the equation: 3=(16^(0.5))−1. For eachsection biased at null, about four output amplitudes and about twophases, e.g. 0 or π, may be generated. The output amplitudes may then becombined by an optical combiner to generate the complete 16QAM signal.Signals with varying levels of modulation may be generated according toembodiments of this disclosure, e.g. 64QAM signal and 16QAM signal.

FIG. 4 is a graph 400 of a 64QAM signal constellation according to anembodiment of this disclosure. FIG. 4 may be generated substantiallysimilar to FIG. 3, however FIG. 4 may use a greater number of sectionsN. Accordingly, higher levels of modulation, e.g. higher order nQAMsignals, may be achieved by adding more sections N to each arm of atransceiver, such as transceiver 200 shown in FIG. 2. For example, usingseven equal sections may enable a transceiver to generate a 64QAM signalaccording to the equation: 7=(64^(0.5))−1.

FIG. 5 is a graphical representation of an embodiment of 16QAMgeneration in one arm of a transceiver using three modulation sections.Varying a value of a digital electrical signal, e.g. signal input 210a-n, being sent to the drivers for driving the modulator may vary aphase of the resulting modulated optical signal. The electrical fieldamplitude (E) is: E=2E0*cos (φ). The phase of the electrical field isdetermined based on the phase of the modulated signal according to theequation: 0 if φ<0 and π if φ>0, where φ is the phase of the modulatedsignal. The signal from one arm of a transceiver, e.g. an I signalcomponent arm, as shown in FIG. 5 may then be combined with acorresponding signal from another arm of the transceiver, e.g. a Qsignal component arm, to form a complete optical signal that may betransmitted to another device.

FIG. 6 is a table presenting electrical field values for 16QAMgeneration using three equal sections according to an embodiment of thisdisclosure. FIG. 6 may be generated substantially similar to FIG. 5.When digital electrical signal inputs received from a digital signalprocessor and/or ASIC, such as digital electrical signal inputs 210 a-nare varied, a resulting electrical field and its associated phase may bedetermined according to the equations discussed above with respect toFIG. 5. For example, in an embodiment of a modulation scheme wherein afirst digital electrical signal D0 is set to −1, a second digitalelectrical signal set to −1, and a third digital electrical signal setto −1, the modulator may have a state of −3 and produce an electricalfield with a value of about 2 and a phase of about 0, as is shown inFIG. 6. In an alternative embodiment wherein the first and seconddigital electrical signals remain the same but the third digitalelectrical signal is set to 1, the modulator may have a state of −1 andproduce an electrical field with a value of about 1 and a phase of aboutπ/3. A non-inclusive sampling of other possible modulation schemes andrelated values is shown more fully in FIG. 5.

FIG. 7 is a schematic diagram 700 illustrating delay in an embodiment ofa transceiver that may be utilized for nQAM generation. The delaybetween optical and electrical signals in a transceiver, such astransceiver 200, may be controlled digitally or by design. An opticaldelay 710 may exist between a first section and a second section of eacharm of a transceiver and have a value of Δt. Accordingly, an opticaldelay between the first section of each arm of the transceiver and thenth section may have a value of ΔNt. A corresponding electrical delay720 may be introduced between the digital electrical signal input 730and logical inverter 740 in each section. The electrical delay 720 maycorrespond to the value of the optical delay at that respective section.The electrical delay 720 may be introduced so that the digitalelectrical signal input 730 may be in phase with the optical signal.

FIG. 8 is a table presenting electrical field values for an embodimentof a transceiver that may be utilized for nQAM generation. The labels ofFIG. 8 may be substantially similar to the labels of FIG. 6. Analternative embodiment of a transceiver for 16QAM generation may beformed by using four equal sections in each arm of the transceiverrather than the minimum number of three sections. However, as shown inFIG. 8, the use of four equal sections may lead to a change in theelectrical field produced by the transceiver and the electrical fieldnot being equally spaced. An unequally spaced electrical field may leadto the transceiver's constellation being distorted. To rectify andequalize the spacing, an embodiment of a transceiver with varyingsegment lengths or biasing at a non-null point may be employed.

FIG. 9 is a schematic diagram of an alternative embodiment of a digitaloptical transceiver 900 that may be utilized for nQAM generation.Transceiver 900 may be substantially similar to transceiver 200;however, transceiver 900 may employ unequal section lengths in each armof the transceiver 900. In other words, each section of transceiver 900may contain a different number of drivers. For example, if a length of afirst section 910 in transceiver 900 is L, the second section 920 may beabout 2*L employing two logical inverters as drivers having the sameinput, the third section (not shown) about 3*L using three logicalinverters as drivers having the same input, and the Nth section 930about N*L using N logical inverters as drivers having the same input.Employing a number of logical inverters about equal to the number ofthat respective section, e.g. one inverter for first section, twoinverters for second section, . . . , N inverters for Nth section, maymaintain an about equal bandwidth for each section. In an embodiment oftransceiver 900, a 16QAM signal may be generated using about twosections in each arm of the transceiver rather than three sections asrequired by transceiver 200 shown in FIG. 2. Generally, the structure oftransceiver 900 may enable it to generate about 2^(N) amplitudes of asignal in both the I and Q modulation sections, thereby generating anabout (2^(N))²QAM signal.

FIG. 10 is a table presenting electrical field values for 16QAMgeneration using two unequal sections according to an embodiment of thisdisclosure. The labels of FIG. 10 may be substantially similar to thelabels of both FIG. 6 and FIG. 8. The values of FIG. 10 may correspondto a hardware structure substantially similar to that of FIG. 9. As canbe seen from FIG. 10, using unequal section lengths may allowtransceiver 900 to generate an equivalent nQAM signal using fewersections than transceiver 200 shown in FIG. 2.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R₁, and an upper limit,R_(u), is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R₁), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. The use of the term about means ±10% of thesubsequent number, unless otherwise stated. Use of the term “optionally”with respect to any element of a claim means that the element isrequired, or alternatively, the element is not required, bothalternatives being within the scope of the claim. Use of broader termssuch as comprises, includes, and having should be understood to providesupport for narrower terms such as consisting of, consisting essentiallyof, and comprised substantially of. Accordingly, the scope of protectionis not limited by the description set out above but is defined by theclaims that follow, that scope including all equivalents of the subjectmatter of the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a digital electricalsignal input; an optical signal input; a digital driver coupled to thedigital electrical signal input; and a modulator coupled to the opticalsignal input and the digital driver, wherein the modulator is configuredto generate a quadrature amplitude modulated signal, and wherein thereis no digital-to-analog converter positioned between the digitalelectrical signal input and the modulator.
 2. The apparatus of claim 1,wherein the digital driver comprises a logical inverter.
 3. Theapparatus of claim 1, wherein the digital driver consists of a pluralityof logical inverters arranged in parallel with each other.
 4. Theapparatus of claim 3, wherein the apparatus does not comprises a linearradio frequency (RF) driver positioned between the digital electricalsignal input and the modulator.
 5. The apparatus of claim 3 furthercomprising: an optical splitter positioned between the optical signalinput and the modulator that splits the optical signal into an Icomponent and a Q component; and an optical combiner positioned betweenthe modulator and an optical signal output that combines the I componentand the Q component.
 6. The apparatus of claim 5, wherein the modulatorcomprises a first section coupled to the I component and a secondsection coupled to the Q component, wherein the first section and thesecond section are coupled in parallel, and wherein a phase shifter iscoupled between the second section and the optical combiner.
 7. Theapparatus of claim 6 further comprising a delay module positionedbetween the digital electrical signal input and the digital driverinput, wherein the delay module is configured to match a phase of theoptical signal input to a phase of the digital electrical signal input.8. The apparatus of claim 7, wherein the apparatus is part of an opticaltransceiver.
 9. An optical transceiver comprising: an optical signalinput; a first modulation section coupled to the optical signal input; asecond modulation segment coupled to the optical signal input andpositioned in parallel with the first modulation section, wherein thefirst modulation section comprises: a first digital electrical signalinput; a first digital driver coupled to the first digital electricalsignal input; and a first modulator coupled to the first digital driver,and wherein the second modulation section comprises: a second digitalelectrical signal input; a second digital driver coupled to the seconddigital electrical signal input; and a second modulator coupled to thesecond digital driver; and an optical signal output coupled to the firstmodulation section and the second modulation section, wherein the firstmodulator section and the second modulator section are configured togenerate a quadrature amplitude modulated signal.
 10. The opticaltransceiver of claim 9, wherein the first digital driver comprises afirst digital logic inverter coupled to the first digital electricalsignal input and the first modulation section, and wherein the seconddigital driver comprises a second digital logic inverter coupled to thesecond digital electrical signal input and the second modulationsection.
 11. The optical transceiver of claim 10, wherein the firstdigital electrical signal input is transmitted from a digital electricalsignal processor to the first digital logic inverter without using adigital-to-analog converter, and wherein the second digital electricalsignal input is transmitted from the digital electrical signal processorto the second digital logic inverter without using a digital-to-analogconverter.
 12. The optical transceiver of claim 10, wherein a linearradio frequency (RF) driver is not used as the first digital driver orthe second digital driver.
 13. The optical transceiver of claim 9,wherein the first modulation section further comprises: a third digitalelectrical signal input; and a third digital driver coupled to the thirddigital electrical signal input and the first modulation section,wherein the third digital driver comprises a plurality of digital logicinverters, and wherein the second modulation section further comprises:a fourth digital electrical signal input; and a fourth digital drivercoupled to the fourth digital electrical signal input and the secondmodulation section, wherein the fourth digital driver comprises aplurality of digital logic inverters.
 14. The optical transceiver ofclaim 9, wherein a bandwidth of the optical transceiver is limited by abandwidth of each respective modulator driver section and not by anoverall modulator value.
 15. The optical transceiver of claim 9, whereinthe optical signal input and each digital electrical signal input have amatched phase.
 16. The optical transceiver of claim 9, wherein a phaseshifter is coupled between the second modulation section output and theoptical signal output.
 17. The optical transceiver of claim 9, whereinthe first modulation section and the second modulation section comprisea segmented optical phase modulator.
 18. A method comprising: receivingan optical signal; and modulating the optical signal using a digitalelectrical signal from a digital electrical signal processor to generatea quadrature amplitude modulated signal, wherein the digital electricalsignal is not converted to an analog signal prior to being used tomodulate the optical signal.
 19. The method of claim 18, wherein a speedwith which the optical signal is modulated is limited by a bandwidth ofthe modulator.
 20. The method of claim 18, wherein the digitalelectrical signal and a digital logic inverter comprise a driver used tomodulate the optical signal.